1. Field of the Invention
This invention relates to a method and apparatus for editing digital signals which are recorded on a record medium and, more particularly, to such a method and apparatus for electronically editing digital signals, such as PCM-encoded audio signals, which are helically recorded on a magnetic tape by a conventional VTR.
2. Description of the Prior Art
In the field of magnetic recording, two types of editing are well-known; physical editing, wherein information recorded on one magnetic tape, such as audio information, and information recorded on another magnetic tape are combined by splicing the two tapes together; and electronic editing, wherein information from a separate source is combined electronically with information previously recorded on a magnetic tape. The physical editing technique generally is used when the information recorded on the magnetic tape is relatively low frequency information, such as recorded audio signals.
Generally, electronic editing is used when the information recorded on the magnetic tape is relatively high frequency information, such as video signals. In electronic editing, edit signals, which may be supplied from a suitable source, such as another record medium, a suitable memory device, or the like, are inserted or assembled with original information. For example, original information on one record medium may be re-recorded onto another record medium until a suitable edit point is reached. Thereafter, the edit information is substituted for the original information, and this substituted edit information is recorded on the second record medium. Subsequently, the original information is recorded once again on the second record medium, resulting in the edit information being "inserted" into the original information. Electronic editing often is used in preparing video tape to be played back or reproduced for broadcast purposes.
Recently, high quality audio recordings have been made by digitally encoding the audio signals to, for example, a pulse code modulation (PCM) format and then recording such PCM-encoded audio signals. For example, left and right channel audio signals are sampled, encoded in PCM form, and the PCM signals are helically recorded on a magnetic tape by a conventional video tape recording or VTR device. However, it is not possible to edit the audio PCM signals in the same manner as has been used for editing video signals while being played back and recorded with VTRs. It is because electronic audio signal editing device must be provided with the following functions:
Firstly, when determining an edit point, it must be possible to detect the location of the edit point with an accuracy that is substantially the same as that possible with an analog recorder; secondly the editing accuracy has to be finer than the frame (or field) unit or finer than 1/100 second; thirdly, it must be possible to remove any discontinuity of the signals at the edit point; and fourthly, the rehearsal must be capable of being carried out with an accuracy substantially the same as that attained upon editing.